Treball final de grau GRAU DE MATEMÀTIQUES Facultat de Matemàtiques i Informàtica Universitat de Barcelona The Seifert-Van Kampen theorem via covering spaces Autor: Roberto Lara Martín Director: Dr. Javier José Gutiérrez Marín Realitzat a: Departament de Matemàtiques i Informàtica Barcelona, 29 de juny de 2017 Contents Introduction ii 1 Category theory 1 1.1 Basic terminology . .1 1.2 Coproducts . .6 1.3 Pushouts . .7 1.4 Pullbacks . .9 1.5 Strict comma category . 10 1.6 Initial objects . 12 2 Groups actions 13 2.1 Groups acting on sets . 13 2.2 The category of G-sets . 13 3 Homotopy theory 15 3.1 Homotopy of spaces . 15 3.2 The fundamental group . 15 4 Covering spaces 17 4.1 Definition and basic properties . 17 4.2 The category of covering spaces . 20 4.3 Universal covering spaces . 20 4.4 Galois covering spaces . 25 4.5 A relation between covering spaces and the fundamental group . 26 5 The Seifert–van Kampen theorem 29 Bibliography 33 i Introduction The Seifert-Van Kampen theorem describes a way of computing the fundamen- tal group of a space X from the fundamental groups of two open subspaces that cover X, and the fundamental group of their intersection. The classical proof of this result is done by analyzing the loops in the space X and deforming them into loops in the subspaces. For all the details of such proof see [1, Chapter I]. The aim of this work is to provide an alternative proof of this theorem using covering spaces, sets with actions of groups and category theory. On this version of the theorem we are going to ask more conditions on the topological space than in the 'classical' proof. Nevertheless, the spaces which do not follow those requirements are a bit 'pathological'. First, we are going to introduce category theory, and all the concepts which will be needed to follow the proof. Then we are going to talk about group actions, focusing on its categorical implications. After we recall the basics of homotopy theory, we are going to see covering spaces and how do they relate with the fun- damental group. Finally, we are going to prove the theorem of Seifert-van Kampen using covering spaces. Acknowledgments I would like to thank Professor Gutierrez for his advice and time given along the making of this paper. I would like also to thank Professors Casacuberta and Vicente Navarro, who were my professors of the topology courses taken in the University of Barcelona, and whose passion and knowledge introduced me to topology. And last but not least, I would like to thank my parents for supporting me everyday with their love and patience. Chapter 1 Category theory In short, category theory studies mathematical structures and its relations in an abstract way. Here we are going to see some basic concepts, but we recommend to read [2] to learn about its logical foundations and [3] for its first steps. 1.1 Basic terminology Definition 1.1. A category C consists of the following: 1) a class Ob(C), whose elements will be called "objects of the category"; 2) for every pair A, B of objects, a set C(A, B), whose elements will be called "morphisms" or "arrows" from A to B; 3) for every triple A, B, C of objects, a composition law C(A, B) × C(B, C) −! C(A, C); the composite of the pair ( f , g) will be written g ◦ f or just g f ; 4) for every object A, a morphism 1A 2 C(A, A) called the identity on A. These data are subject to the following axioms: 1) Associativity axiom: given morphisms f 2 C(A, B), g 2 C(B, C), h 2 C(C, D) the following equality holds: h ◦ (g ◦ f ) = (h ◦ g) ◦ f . 2) Identity axiom: given morphisms f 2 C(A, B), g 2 C(B, C) the following equalities hold: 1B ◦ f = f , g ◦ 1B = g. 1 2 Category theory Examples 1.2. Here is a list of some obvious examples of categories and the cor- responding notation, when it is classical: 1. Sets and functions: Set. 2. Topological spaces and continuous maps: Top. 3. Groups and groups homomorphisms: Gr. 4. Abelian groups and groups homomorphisms: Ab. 5. Commutative rings with unit and ring homomorphisms: Rng. 6. If R is a commutative ring, R-modules and R-lineal maps: ModR. Definition 1.3. A functor F from a category A to a category B consists of the following: 1) a function Ob(A) −! Ob(B) between the classes of objects of A and B; the image of A 2 Ob(A) is written F(A) or just FA; 2) for every pair of objects A, A0 of A, a map A(A, A0) −! B(FA, FA0); the image of f 2 A(A, A0) is written F( f ) or just F f . These data are subject to the following axioms: 1) for every pair of morphisms f 2 A(A, A0), g 2 A(A0, A00) F(g ◦ f ) = F(g) ◦ F( f ); 2) for every object A 2 A F(1A) = 1FA. Remark 1.4. Given two functors F : A ! B and G : B ! C, a pointwise composi- tion immediately produces a new functor G ◦ F : A ! C. This composition law is obviously associative. Examples 1.5. Some examples of functors include: 1. For any category C, the identity functor 1C : C ! C that maps each object and morphism of C to itself. 1.1 Basic terminology 3 2. The "forgetful functor" U : Ab ! Set that maps a group (G, +) to the un- derlying set G and a group homomorphism f to the corresponding map f . 3. Let R be a commutative ring. Tensoring with R produces a functor from the category Ab of abelian groups to ModR: − ⊗ R : Ab −! ModR. An abelian group A is mapped to the group A ⊗Z R provided with the scalar multiplication induced by the formula (a ⊗ r)r0 = a ⊗ (rr0) A group homomorphism f : A ! B is mapped to the R-linear mapping f ⊗ idR. 4. We obtain a functor P : Set ! Set from the category of sets to itself by sending a set A to its power set P(A) and a map f : A ! B to the "direct image map" from P(A) to P(B). Definition 1.6. A morphism f : A ! B in a category C is called an isomorphism when there exists a morphism g : B ! A of C which satisfies the relations f ◦ g = 1B, g ◦ f = 1A. Proposition 1.7. Every functor preserves isomorphisms. Proof. Let F : A ! B be a functor and f : A ! B an isomorphism in A. Let g be a morphism such that f ◦ g = 1B, g ◦ f = 1A. Then 1FA = F(1A) = F(g ◦ f ) = F(g) ◦ F( f ), 1FB = F(1B) = F( f ◦ g) = F( f ) ◦ F(g). Definition 1.8. Consider two functors F, G : A ⇒ B from a category A to a cate- gory B. A natural transformation a : F ) G from F to G is a class of morphisms (aA : FA ! GA)A2A of B indexed by the objects of A and such that for every morphism f : A ! A0 in A, the following diagram commutes aA FA / GA F f G f FA0 / GA0, aA0 4 Category theory that is aA0 ◦ F( f ) = G( f ) ◦ aA. A natural isomorphism is a natural transformation a : F ) G in which every arrow aA is an isomorphism. In this case, the natural isomorphism may be depicted as a : F =∼ G. Definition 1.9. An equivalence of categories consists of functors F : A B : G together ∼ ∼ with natural isomorphisms a : 1A = GF, b : FG = 1B. Two categories A and B are equivalent, written A =∼ B, if there exists and equivalence between them. ∼ Proposition 1.10. Consider two functors F, G : A ⇒ B such that a : F = G. If F is an equivalence of categories, then so is G. Proof. Using the notations of 1.8 we have a commutative diagram aA FA / GA F f G f FA0 / GA0, aA0 ∼ where the aA are isomorphisms. Denote H the functor such that b : 1A = HF ∼ and g : FH = 1B. Composing the above diagram with H and considering the one given by b gives the following commutative diagram bA HaA A / HFA / HGA 1A f = f HF f HG f A0 / HFA0 / HGA0, bA0 HaA0 with bA, bA0 , HaA and HaA0 isomorphisms. So, by the outer square of the diagram 0 ∼ 0 ∼ we see b : 1A = HG. We show in a similar way that g : GH = 1B. Definition 1.11. Consider a functor F : A ! B and for every pair of objects A, A0 2 Ob(A), the map A(A, A0) −! B(FA, FA0), f 7! F f . 1) The functor F is faithful when the above-mentioned maps are injective for all A, A0. 2) The functor F is full when the above-mentioned maps are surjective for all A, A0. 3) The functor F is essentially surjective (on objects) when each object B 2 Ob(B) is isomorphic to an object of the form FA, A 2 Ob(A). 1.1 Basic terminology 5 Theorem 1.12. Assuming the axiom of choice, given a functor F : A ! B the following conditions are equivalent: 1) F is full, faithful and essentially surjective. 2) F is an equivalence of categories, i.e., there exist a functor G : B ! A and two natural ∼ ∼ isomorphisms a : 1A = GF, b : FG = 1B. Proof. See [3, Theorem 1.5.9].